In the machine processing of various types of tickets, tags, labels, postage imprints and the like, it is generally know to employ detectors which are responsive to colors, and in many cases to the fluorescence of an ink which may be excited by ultraviolet light. Fluorescent inks and dyes and methods of making the same have long been known as disclosed in U.S. Pat. Nos. 2,681,317; 2,763,785; 3,230,221; 3,421,104; 3,452,075; 3,560,238; and 4,015,131. It is known, for example, in the postage meter art to provide a red fluorescent ink for the machine reading of processed mail. Fluorescent inks are those in which the ink exhibits a first color, such as black or green, in the visible spectrum and a second color when subjected to ultraviolet light.
Although the inks and dyes used previously for the purposes described have worked relatively well, certain drawbacks exist. With regard to fluorescent inks, prior fluorescent inks were dispersion inks that experience frequent color separation upon aging. In addition, the prior red fluorescent inks with non-red visual color generally have dull visual colors. It is difficult to obtain a red fluorescent ink with a non-red visual color in solution because of interaction between the dyes that causes the non-red visual color to absorb the fluorescent imaging. This is known as quenching and is occasioned by many factors such as internal conversion, competing mechanisms, absorption of incident light by foreign molecules, solvent interaction, and the like. The quenching effect is obvious with the blue dyes that absorb light between 600-640nm which is the emission range of the red fluorescent dye.
In a dispersed system with solute particles &gt;5 .mu. the quenching effect is not a problem because there is weaker interaction between the dye/pigment components as a result of low collision probability.
Clearly, it would be advantageous to provide a combination of materials that yield a red fluorescent color when subjected to ultra violet light and a non-red visible color. It is also desirable to have a fluorescent ink in solution that will not separate or become dull with the passage of time. Throughout the balance of this specification, red fluorescent solution ink with a non-red visual color will be referred to as non-red solution ink. Of course, what is meant by visual color is a color visible in daylight or white light.